1. Technical Field
This invention applies generally to plasma etching of semiconductor materials and more particularly to a novel process and apparatus for maintaining the gap between a consumable upper electrode and a lower electode, thereby, maintaining a more effective etching rate.
2. Description of the Prior Art
The following three documents relate to various methods dealing with moveable electrodes in plasma etch systems.
U.S. Pat. No. 5,354,413 issued Oct. 11, 1994 to Smesny et al, discloses an electrode position controller for a semiconductor etching device that is calibrated each time the etching device is turned on.
U.S. Pat. No. 5,344,542 issued Sep. 6, 1994 to Mahar et al, teaches a plasma etch system that has electrodes that are moveable so as to provide a selectable gap for either a single or multiple step processing modes.
U.S. Pat. No. 5,336,355 issued Aug. 9, 1994 to Zarowin et al, shows a method and apparatus for confinement of a plasma etch region for precision shaping or contouring surfaces of substances and films.
It is well known that during the manufacturing of silicon-based semiconductor devices the goal of the contact or via etch processes is to obtain a minimum dimension, high aspect ratio hole in silicon dioxide with straight walls, and selectivity to etch silicon or polysilicon.
The gas ratio of F:C (Fluorine:Carbon) limits the selectivity of silicon dioxide to etch either silicon or polysilicon during the etching process. Varying this ratio directly affects selectivity. Fundamentally, selectivity can be increased by reducing the F radical concentration or by increasing the C radical concentration. The fluorine atoms contribute to a faster etch rate on silicon than on silicon dioxide thereby reducing selectivity. Carbon atoms, on the other hand, operate as a polymer source that slows the etch rate thus increases selectivity. It has been found that selectivity can be increased significantly by adding H.sub.2 which reacts with F to form HF so that the F concentration in process is reduced. It is also found that selectivity is reduced by adding oxygen, which combines with carbon from reaction gas of CF.sub.3 and liberates F so that the F radical concentration is increased.
In addition to adding a different gas to achieve a desired selectivity, the same result is achieved by using a commercially available electrode made from either silicon or graphite materials for varying the F:C ratio. Silicon works to dissipate the F radical concentration while graphite provides the carbon source to vary the F:C ratio. The electodes erode with time thus affecting the etch rate of silicon dioxide. The rate may increase or decrease as the electrode is consumed.
The gap dimension and parallism between the two electrodes are critical calibration steps that are performed manually during machine maintenance. This procedure is sufficient for non-consumable electrodes due to its planar and smooth surface, however, when using consumable electrodes, the profile erodes forming a concave shape with enlarged gas distribution holes thereby precluding accurate calibration.